Cryoablation techniques eradicate arrhythmogenic tissue by inducing hypothermia, necrosis, and apoptosis through the application of freezing temperatures to the target cardiac tissue. An unintended consequence of cryoablation, particularly when ablating tissue near the right-sided pulmonary veins in the left atrium, is the attenuation of phrenic nerve function due to the freezing temperatures permeating through the cardiac tissue and into the phrenic nerve. This phenomenon is also a concern when ablating nearby tissue using non-cryogenic ablation modalities, such as radiofrequency ablation. The phrenic nerve is made up mostly of motor nerve fibers that produce contractions of the diaphragm and thus affect breathing and respiration patterns and conditions. In addition, the phrenic nerve provides sensory innervation for many components of the mediastinum and pleura, as well as the upper abdomen, especially the liver, and the gall bladder. Injury to the phrenic nerve may severely impact normal respiratory function and can require many weeks or months to resolve. In the worst cases, this reduced function requires mechanical ventilation assistance to maintain respiration. This side effect of cryoablation can manifest as a transient phrenic functional block, transient phrenic nerve palsy (PNP), or longer-term phrenic nerve injury.
The potential for phrenic nerve injury can be significantly reduced through the use of phrenic nerve monitoring during ablation procedures. By pacing the phrenic nerve superior to the ablation site and monitoring the amplitude of compound motor action potentials (CMAP), the integrity of the phrenic nerve can be continuously assessed during ablation. If cryoablation energy is removed at the first sign of phrenic impairment, the injury is in almost all cases transient, with normal phrenic function returning within minutes.
Currently, phrenic monitoring is typically done using one or more of several possible methods, such as pacing the phrenic nerve and using continuous fluoroscopy during the ablation to visualize a consistent diaphragmatic response, palpitating the abdomen to confirm diaphragmatic movement, intercardiac echocardiography (ICE imaging), or fetal heart monitoring. However, these methods all require vigilance on the part of the operator, and can distract the physician from the main focus of ablating tissue. Additionally, in the case of fluoroscopic monitoring, the patient is exposed to increased x-ray radiation.
It is therefore desirable to provide an automated method of monitoring phrenic nerve function. This automated method would reduce physician distraction, reduce procedure fluoroscopy time, and ensure timely identification of transient injury, leading to prevention of long-term phrenic injury.